To understand the role of living organisms in the water cycle, you
must first understand the general processes of the water cycle and its
non-living components. The three main steps of the cycle are
condensation, precipitation and evaporation.
Condensation is the change from a gas to a liquid; this occurs in the
atmosphere under conditions of low temperature and pressure, resulting
in the formation of clouds. Precipitation occurs when the water droplets
formed among the clouds become large and heavy and fall to the ground,
most commonly in the form of rain. Other forms of precipitation include
snow, sleet and hail, and depend upon other atmospheric conditions.
The final step of the cycle is evaporation, during which water from
the Earth's surface evaporates into gas, which rises back into the
atmosphere. The greatest sources of evaporation are large bodies of
water, such as lakes and the ocean. Living organisms, mainly plants and
animals, also contribute to this last step of the cycle through
processes, such as respiration, perspiration and transpiration.
